1. Field of the Invention
The instant invention relates to electrically programmable read only memories (PROM's), and more particularly, the instant invention relates to PROM's which use a vertical fuse technique and to processes for their manufacture.
2. Prior Art and Technical Considerations
There are two general types of PROM devices, i.e., the irreversible fuse-link programming device and the trapped-charge device, generally referred to as an erasable PROM or EPROM. With the erasable PROM, the trapped charges are removed by exposing the integrated circuit chip to an erase voltage or an ultraviolet light. Memory is maintained by a charge stored in an isolated conductor or by a charge held in deep trap states, such as may exist at the interface of dielectric heterostructures. However, while such EPROMs may be erased and reprogrammed, they require a high programming voltage and must maintain extremely rigid dielectric quality standards in order to retain information for a commercially acceptable period of time, which is perhaps ten years. In EPROM devices, considerable manufacturing testing must be utilized to insure that the charges are actually retained and that the devices are reliable. This additional testing contributes greatly to the cost of trapped-charge memories. PROM's utilizing irreversible, fuse programming techniques have demonstrated better retention periods than EPROMs but heretofore have been even more expensive to fabricate than EPROMS. In many cases, the most important factor is the cost per bit for a user programmable medium.
While it has been generally recognized that fuse-type PROMs potentially have advantages in reliability over trapped-charge devices, the prior art in this area has been unsuitable for low cost applications. Until very recently, fuse type PROMs have been manufactured utilizing bipolar processes. However, bipolar manufacturing processes are expensive, and the resulting devices are of low density and consequently higher cost. Moreover, the peripheral circuitry necessary for bipolar devices consume a lot of power. This has, in most instances, made fuse devices less desirable than their metal oxide silicon (MOS) trapped-charge counterparts. An exception to this is where long term reliability and high speed are required, since bipolar structures have memory-read access times five times faster than MOS structures.
Early bipolar PROM devices consisted of a simple diode/fuse arrays with bipolar peripheral transistors. These transistors could supply sufficient current and voltage to open-circuit selected fuses for programming. While this approach achieves high density, because of the simplicity of diode fabrication, it is not well suited to large memory arrays since it requires the fabrication of two levels of high current conductors traversing the long distances present in VLSI memory layouts. The presence of an active, high gain device at each node of the memory array will reduce the stringent current-carrying requirements for a level of the interconnect and has therefore been generally adopted by manufacturers of higher density PROMs.
The need for an inexpensive, high density, programmable ROM has resulted in several innovations in active array, fuse-type PROM devices. The SEMICONDUCTOR CMOS DIGITAL PRODUCTS DIVISION of the Harris Corporation has built a PROM part (HM 6641) utilizing CMOS technology which avoids the power-intensive nature of the peripheral circuitry necessary with bipolar devices. In their technique, lateral polysilicon fuses are connected to separate NPN bipolar transistors in the array, thus gaining the advantages of a CMOS process while maintaining the fuse-blowing power of bipolar devices. However, their devices are essentially low density devices, because they utilize lateral fuses which are placed adjacent to their bipolar transistors. In addition to low density, the Harris devices have the disadvantage of the need for an opening in the passivation layer to allow the expulsion of the blown fuse material. These holes in the passivation layer result in an inherent reduction in reliability.
Another example of recent PROM technology is exemplified by the article "Fujitsu Comes Out with Fastest 64K PROM", Electronic Engineering Times, Dec. 6, 1982, Page 10. The designers in that situation utilize a completely vertical configuration, but do so at the expense of bipolar technology. This approach is further discussed in an article by Dave Burskey, Ed., "Isolation Process and DEAP Technique Cut PROM Access Time", Electronic Design, Vol. 27, No. 14, July 5, 1979. DEAP is an acronym for "Diffused Eutectic Aluminum Process". In essence the Fujitsu designed DEAP programming technique utilizes an aluminum and polysilicon layer sitting on top of an emitter-base blocking diode. During programming, a reverse current is applied to the emitter surface to diffuse aluminum-silicon eutectic down through the emitter-base blocking diode junction. This shorts the junction. The Fujitsu arrangement utilizes a functional PNP transistor for a program device, while unprogrammed devices remain as four-layer NPNP devices to block current flow. While the Fujitsu process has the advantages of increased density due to a vertical fuse stacked on top of an active device, the Fujitsu process requires full isolation bipolar technology and uses bipolar transistors for peripheral circuitry. The resulting structure is expensive to fabricate and has a high level of power consumption.
An improved bipolar PROM with a similar vertical blocking diode which is shorted during programming has been described in U.S. Pat. No. 4,403,399 by Taylor. This invention discloses a vertical diode/bipolar transistor structure which does not require device isolation and therefore may have improved density over the Fujitsu DEAP technology. However, this device is programmed by creating an extremely high power density region in a thin epitaxial layer to induce migration of aluminum particles, therefore shorting the blocking diode junction. The bipolar devices in the array provide some current gain but their effectiveness is degraded by the high programming currents required to short the blocking diode junction since the bipolar transistor gain is low at such high current. Greater current could be obtained from the bipolar transistor by increasing the emitter area; however, because the size of the blocking diode is determined by the emitter size a larger diode requires a proportionately larger programming current. For this configuration, most of the programming current must be supplied as base current to the bipolar transistors from the peripheral circuitry. Therefore, this device is best built in a bipolar process with bipolar peripheral circuitry and two levels of low resistivity interconnect.
Another approach is set forth in an article by Tanamoto et al., "A Novel MOS PROM Using Highly Resistive Polysilicon Resistor", IEEE Transactions on Electron Devices, Vol. ED-27, No. 3, Pages 517-520, March, 1980. This PROM utilizes a vertical anti-fuse structure and contains only MOS circuitry. The anti-fuse consists of high resistivity polysilicon which undergoes a dramatic decrease in resistivity upon application of sufficient voltage and current. The current required to lower the resistivity of the polysilicon is much lower than required to short the blocking diode in the devices described above. The current is lower because a different physical mechanism is employed, i.e. a memory switching phenomenon which is characteristic of polysilicon and calcogenide glasses. This phenomenon has been studied and described by Mahan in Applied Physics Letters Vol. 41, p. 479, Sept. 1, 1982. Consequently, it is feasible to make such a PROM in an MOS process and gain the advantage of low power consumption, lower cost and reduced process complexity. Since this approach utilizes a vertical anti-fuse, the lateral area consumed by each bit is reduced when compared to a lateral fuse. With this approach, however, a MOS transistor is required at each bit in the array. The MOS transistor used with each cell is an inherently lateral device adjacent to the fuse which lowers the density of the entire structure, even though the fuse is vertical.
In view of the deficiencies of the prior art, there remained a need for high density, fuse type PROMs which require low programming power, are low in manufacturing cost and reliable as to the data entered.